An object of the present invention is to provide an engine having an exhaust gas treatment apparatus attached thereto, wherein an exhaust pipe 13 can be prevented from being damaged by a vibration of the engine body 1. To achieve the object, in the engine having the exhaust gas treatment apparatus attached thereto in which an exhaust gas treatment apparatus 4 is supported by an engine body 1 and the engine body 1 and the exhaust gas treatment apparatus 4 communicate with each other through an exhaust pipe 13, the casing of the exhaust gas treatment apparatus consists of a plurality of casing units 5, 6, and 7 separable from each other, connection flanges 8, 9, 10, and 11 are disposed in respective end portions of the casing units 5, 6, and 7, the casing units 5, 6, and 7 are coupled together by a connection of the connection flanges 8, 9, 10, and 11, and the exhaust gas treatment apparatus 4 is supported by the engine body 1 through the connection flanges 8, 9, 10, and 11.